Oyster Species of the Sub Tropical Coast of Pakistan (Northern Arabian Sea)

Indian Journal of Marine Sciences Vol. 31(2), June 2002, pp. 108-118

Oyster species of the sub tropical coast of Pakistan (northern Arabian Sea)

Ghazala Siddiqui & Muzammil Ahmed Centre of Excellence in Marine Biology University of Karachi, Karachi-75270, Pakistan [ E-mail: [email protected] ]

Received 11 April 2001, revised 1 January 2002

In the present study 9 species of oysters belonging to three genera (Crassostrea, Saccostrea and Ostrea) were identified on the basis of their conchological and malacological features. These included Crassostrea gryphoides, C. madrasensis, C. belcheri, C. glomerata, Saccostrea cucullata, S echinata, Ostrea nomades, O. folium and O. cristagalli. Two species Ostrea nomades and O. cristagalli which are being reported for the first time from Pakistan. Crassostrea gryphoides and C. rivularis from Hub River Delta were earlier treated as separate species have been assigned to a single species C. gryphoides. Crassostrea gryphoides and C. madrasensis, which occur in the same habitat and show marked similarity in their external shell morphology, but differ in the colouration of their adductor muscle scar, are treated here as separate species.

[ Key words : Taxonomy, oyster, Crassostrea, Saccostrea, Ostrea ]

The coast of Pakistan, which is confined to the north- 1992 to December 1993. Collections from Buleji ern Arabian Sea, lies outside the tropics, as its south- rocky ledge were made from July 1992 to December eastern boundary is only a few miles short of Tropic 1993. From Buleji sampling was also done in April of Cancer. The southeastern extremity touches the 1994. The oysters were also collected from Paradise India with whom Pakistan shares a more or less com- Point, Karachi from January 1992 to May 1992. mon oyster fauna. The collection sites are shown in Fig. 1. Hub Delta The information regarding the taxonomy of Paki- is mostly sandy but the mouth of river forms sandy- stani and Indian oysters is based on the several re- cum-muddy platforms, which are fairly easy to walk ports1-23. From Pakistan, 4-10 species of oysters were recognized and were placed in genus Ostrea24-26. Ahmed27 updated the nomenclature and placed the oysters under two genera, Crassostrea and Ostrea, recognizing six species of Crassostrea, (C. rivularis, C. viriginica, C. gryphoides, C. tuberculata, C. glomerata and C. quercina) and one species of Ostrea, (O. folium). Later C. gryphoides and C. rivularis were placed in single species C. rivularis; and C. glomerata and C. tuberculata in the genus Saccos- trea.28 Ahmed et al.29 from Gwadar (Balochistan coast) included the tuberculated oysters like C. tuberculata and C. echinata in the genus Saccostrea. The present paper reports the presence of nine species of oysters by attempting to update their nomenclature. Materials and Methods Oysters were collected from three localities, i.e, Hub River Delta, Keamari Seawall (Karachi Harbour) Fig. 1 ⎯ Collection sites* of oysters along Sindh and Balochistan and Buleji. From Hub Delta the collections were Coasts. (♣ Crassostrea gryphoides; U C. madrasensis; made from August 1992 to December 1993, from 8 C. belcheri; Ä C. glomerata; ♠ Saccostrea. echinata; ◗ S. Keamari the samples were obtained from October cucullata; ▼ Ostrea nomades; O. folium; ? O. cristagalli). Siddiqui & Ahmed : Oysters of Pakistan 109

on. The delta has been cut off now from the main Ahmed27, Thomson31 and Stenzel32. Other publications river due to the recent construction of Hub Dam. The were also consulted33, 34 for the sake of comparison. seawater now extends two km inland from the Sea. The identifications of the oyster species were based The beds of C. gryphoides and C. madrasensis are on the conchological characters i.e; morphology of now virtually on the verge of disappearance from the shell, its colour and luster, absence or presence of here. The salinity here ranges from 32 to 40 ‰. Buleji denticles and shape, colour and position of adductor rocky ledge is a triangular platform extending into the muscle scar. Some malacological features like pres- open Arabian Sea. It is uneven in profile, with its ence or absence of promyal chamber, colour of the right flank (facing the sea) lying a bit low and facing mantle folds and tentacles of the mantle folds were maximum wave action. In this wave swept part a also examined. The morphological features used for luxuriant growth of seaweeds and zooanthid coelen- the identification of the oyster species are shown in trates was observed. The middle part of this ledge is Fig. 2 B, C. All specimens were examined alive ex- elevated at places or lies low at other parts making it a cept where indicated otherwise. somewhat backwater-like protected environment. The left margin of the ledge has much less wave action Results than the right one. In the main body of the triangular Phylum : Mollusca ledge exist small and large pools of water in which Class : Bivalvia occur cerithid snails in abundance as well as floating Subclass : Lamellibranchia algae and pelagic sea life. In the small shallow pools Order : Anisomyaria of the low tide area occurs the small larviparous oys- Family : Ostreidae ter O. nomades. This oyster also occurs on the margin Genus : Crassostrea of the pools covered with a film of silt which blends with the substratum below. The salinity here mostly in Genus: Crassostrea (Sacco) 31 the range of 35 to 40 ‰. The Keamari seawall is an Description of the genus as given by Thomson is artificial breakwater wall, about 1.5 km long, extend- as follows . “Shell elongate, very variable shape even ing into the open Arabian Sea from Karachi Harbour within the species; lower valve deep, cup-shaped, re- (Fig. 1). It is directly affected by oil pollution emanat- cessed under hinge; the muscle scar displaced both ing from the oil terminal located in Karachi Harbour. towards the dorso-lateral edge and towards the lip; Water is relatively clear and unturbid and salinities upper valve opercular, flat except sometimes upraised are generally in excess of 35 ‰ for much of the year. at the rim, may or may not have a row of denticles Paradise Point is a wave-lashed rocky shore luxuriant along the rim laterally from the hinge. A promyal in algal growth, many species of gastropds, zooanthid chamber on the right side. Non-incubatory”. and coelentartes are found here. The water is clear and the salinity ranges between 35 to 40 ‰. Saccos- trea cucullata is found blended with the substratum at a tidal height of 2-3 m. Coast of Gwadar is a T-shaped formation. Different species of gastropods, bivalves, crustaceans, coelentrates, polychaetes, echinoderms, tunicates and seaweeds are found abundantly on East and West Bays of Gwadar. The West Bay is a unique shore with four species of oysters occurring on the same boulders29. The oyster species occur at tidal height of 1.5 to 2.5 m. The oysters were measured to the nearest millime- ter by the method described by Galtsoff30 (Fig. 2A). In different sizes of Crassostrea gryphoides and C. madrasensis the hinge size was also measured from the tip to the centre of the hinge axis. The curvature of hinge to the left or right was also noted. The species Fig. 2 ⎯ A) Diagram showing the regions where height and length were measured. (After Galtsoff, 1964). B) Morphological features were identified with the help of the reports of: Awati used for the identification of oyster species, and. C) A portion of 11 18 20,21 28 & Rai , Durve , Patel & Jetani , Mane , mantle fold showing middle and inner mantle folds.

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Species: Crassostrea gryphoides (Schlotheim) (Fig. 3) tion/uplifting of the oyster habitat out of water. The coast of Balochistan, in Pakistan, is well-known for Synonyms: Ostrea gryphoides Awati & Rai11, the uplifting of its terrain so that 10.26-12.82 m high Rai12, Durve17 Kazmi24, Ranjha25, Hasan26, Vreden- cliffs stand at the back of the tidal zone29 bearing a burg35; Crassostrea gryphoides Durve18, Patel & Jet- rich record of fossil marine fauna on top38. ani21, Ahmed27, Rao36 ; Crassostrea rivularis Asif28; Remarks: Earlier Asif28 reidentified this species as Crassostrea cattukensis Nagabhushanam & Mane22 . C. rivularis from Sonari (Balochistan) near Hub River In this study 685 oysters were examined from Hub Delta and indicated that the older specimen of River delta (size 12.9 – 260 mm ). Irrespective of the C. rivularis resemble the characteristics described for size of its available specimens this species showed C. gryphoides27. We find that all these oysters irre- elongated shells, anterior regions narrow and posterior spective of their sizes belonged to the species C. gry- region broad; lower valve slightly concave, heavy and phoides which were earlier identified as O. gry- thick specially below the hinge area; upper valve flat phoides (=C. gryphoides)11,17 and subsequently and less thick, forming a lid over the lower valve; C. gryphoides27. We are of the opinion that C. rivu- oysters from 41 to 240 mm size showed hinge area laris is an altogether different species, that occurs in long with a groove curving to the left in 68 % and to Japan (Numachi39) and not in Pakistan. Hence the the right in 32 % of the oysters examined; a well oyster specimens which were earlier identified as marked recess below the hinge area present in all the C. rivularis by previous authors certainly belong to specimens, the hinge area gradually increases with C. gryphoides. increase in shell size in different sizes of oysters examined. Variations in shell shape in some oysters Species: Crassostrea madrasensis (Preston) (Fig. 4 A,B) were likely due to overcrowding and orientation. Promyal chamber present; colour of the inner mantle Synonyms: Ostrea cucullata Hornell4; Ostrea ma- 3 11 17 28 fold in 97 % specimens pigmented black and with two drasensis Preston , Awati & Rai , Durve , Ranjah ; rows of tentacles; in the remaining oysters it was yel- Ostrea virginiana Prashad10, Moses40; Crassostrea 18 19 21 low; tentacles of the middle mantle fold in 77 % madrasensis, Durve Rao , Patel & Jetani ; specimens were yellow and in the remaining black Crassostrea virginica, Ahmed27; Crassostrea cat- pigmented. tukensis, Nagabhushanam & Mane22 , Stenzel32. Habitat: This species occurs at about 0-tide level in In this study 47 oysters were examined from Hub the creeks, estuaries, backwaters and muddy low tidal River Delta (size 35-180 mm). This species shows areas of Sindh and Balochistan. At Hub River Delta close resemblance to C. gryphoides (Table 1) in shell where live oyster reefs were found fifteen to twenty form, except that the inner margins of both valves and years ago37, oysters are now on the verge of extinc- adductor muscle scar are of purple colour and the tion, since mostly oyster boxes can be seen there. This recess beneath the hinge is more well marked and may have been partly due to the result of the construc- more pronounced than in C. gryphoides. The hinge tion of Hub Dam or due to the gradual eleva- area is like that of C. gryphoides, being long, with a

Table 1 ⎯ Comparison of the shell characteristics of two species of Pakistani oysters (Crassostrea gryphoides and Crassostrea madrasensis) with the American oyster Crassostrea virginica.

Shell Species characteristics C. gryphoides C. madrasensis C. virginica Upper valve Flat, lid like, about equal to lower Flat, lid like about equal to lower valve Flat, lid like, smaller than the valve lower valve Lower valve Thick, heavy, concave, deeply Thick, heavy, concave, more deeply Thin, deeply cupped. recessed beneath the hinge area. recessed beneath the hinge area. Blotches of purple colour present towards the margins of the valve. Hinge Long, deeply grooved, curved to Long, deeply grooved curved to right or Short, deeply grooved curved right or left. left. to left. Adductor White, roundish or bean-shaped, Purple, roundish or bean-shaped, dor- Purple, reniform, posterio- muscle scar dorsally placed. sally placed. dorsally placed. Siddiqui & Ahmed : Oysters of Pakistan 111

Fig. 3 ⎯ A) Crassostrea gryphoides (a) outer view of upper valve (b) inner view of lower valve. B) C. gryphoides: lateral view. C) C. gryphoides: (2) C. madrasensis: (a) outer view of upper valve (b) inner view of lower valve. D) C. gryphoides: morphological variation in shell. E) C. gryphoides: inner view of shell.

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deep groove, and curved to left in many more shaped; hinge broad, forming a small beak at the ante- specimens (65 %) than to right (35 %). The hinge area rior end, ligament brown in colour; cavity beneath the gradually increases with shell size. In the soft parts, hinge very shallow; anterior end of the shell is some- the promyal chamber is present and the colour of the what narrow compared to the posterior side, ventral inner mantle fold in 85 % specimens was pigmented margin slightly expanded. The exterior colouration of black with two rows of tentacles while in the the shell is dull green. Internally the shell is white and remaining specimens it was yellow. Tentacles of the sometimes nacreous with no denticulations; upper middle mantle fold in 80% individuals were yellow shell margin has a narrow band of greenish brown and in the remaining 20 % were black. colour. The adductor muscle scar is white being oblong lunate towards the hinge side; slightly dis- Habitat: Same as C. gryphoides in the creeks of Sindh placed dorsally. A promyal chamber is present. The and Balochistan. colour of the inner mantle fold is black. The mantle Remarks: Crassostrea madrasensis was found in bears two rows of tentacles; the middle folds being the same habitat and at the same tidal level as C. gry- usually yellow, sometimes black pigmented. phoides. It occurs in the Hub Delta but in much small Habitat: This species was found on stones of the numbers (6 %) and cannot be separated from C. gry- Keamari seawall, semi-protected habitat, at tidal phoides on the basis of its external shell morphology. heights of about 0-0.9 m near the mouth of the Internal morphology of its shells differs, with respect Manora channel in Karachi Harbour. The open Ara- to the purpleness of the adductor muscle scar and in- bian Sea begins at a distance of about 1 km from here. ner surface of the shells (Table 1). Earlier workers This species seems to be tolerant of the polluted envi- had suggested that O. madrasensis is closely related ronment of the channel. This species was earlier re- to the American oyster Ostrea virginiana (=Crassostrea ported from the littoral zones of Karachi, northern virginica)5,11,35. Prashad10 separated the Indian and bank of Korangi creek and Mekran coasts11,24,26. American species and assigned the Indian and Suma- Remarks: The shell characteristics are similar to tran forms to Ostrea gryphoides. Ahmed27 identified those described by earlier workers11,20,21, but the pur- this species from Pakistan as C. virginica, but consid- plish green scales reported by them do not occur here ers it an error now (in this paper). in this oyster from Keamari seawall. The upper sur- During the present study the specimens of C. ma- face of the right convex valve of this species always drasensis from the Hub Delta were compared with presents a dull greenish appearance. specimens of the American oyster C. virginica ob- Species: Crassostrea glomerata (Gould) (Fig 4D): tained from USA by Ahmed and were found different (Table 1). Crassostrea madrasensis resembles C. gry- Synonyms: Saxostrea amasa Iredale43; Crassostrea phoides closely and would be inseparable on the basis amasa Iredale (in Carreon33); Ostrea glomerata Awati of the outer appearance and morphology of their & Rai11; Hasan26; Saccostrea glomerata Asif 28; shells. It is probable, however, that C. madrasensis Crassostrea glomerata Ahmed27. found in Pakistani waters may simply be a morpho- In this study 50 specimens were examined from logical variant of C. gryphoides. Durve17 suggested Karachi (size 40-70 mm). The shell is dull brown, that the slight variation in the shape of shell and the tainted with purple; internally the shell is white with adductor muscle scar of the two species may be ra- purple margins. In most of the specimens a character- cially or environmentally influenced. However, been istic purple band is present around the margins of the able to separate the two species on the basis of their lower valves, which have a deep upward ascending protein band pattern (paper in preparation). recess under the hinge. The lower valve is laminated with its margins sometimes projecting into spout-like Species Crassostrea belcheri (Sowerby) (Fig 4 C): processes. The upper valve is opercular, without den- Synonyms: Ostrea discoidea Rai12; Ostrea belcheri ticulations and slightly smaller than the lower valve. Awati & Rai11, Kazmi24, Hasan26; Crassostrea The adductor muscle scar is purple, located dorsally, belcheri Patel & Jetani21, Chen41, Kong & Luh42. bean shaped and with a slight concavity facing the In this study 233 specimens were examined from hinge area. The hinge is narrow, grooved and forms a Keamari seawall ( size 9-171 mm). The shell is large, small beak. more or less rounded to oval in shape, and foliaceous, Habitat: This species usually occurs in protected thick, lower valve flat, upper valve convex, dome environment, at about 1.5-3.0 m tidal height27. It can Siddiqui & Ahmed : Oysters of Pakistan 113

Fig. 4 ⎯ A,B) Crassostrea madrasensis: morphological variations in shell form. (a) upper valve (b) lower valve. C) Crassostrea belcheri : (a) inner view of lower valve; (b) outer view of upper valve. D) Crassostrea glomerata : (a) inner view of upper valve (b) inner view of lower valve.

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Fig. 5 ⎯ A) Saccostrea echinata: (a) outer view of upper valve; (b) inner view of lower valve; (c) inner view of upper valve; (d) whole shell. B) Saccostrea cucullata:: outer (a) and inner (b) view of upper valve; (c)inner view of lower valve. C) Saccostrea cucullata: (a) inner view of upper valve (b) inner view of lower valve. D) Ostrea nomades: (a) outer view of upper valve; (b)inner view of upper valve: (c) inner view of lower valve. E) Ostrea folium: (a) inner view of lower valve. (b) inner view of upper valve. F) Ostrea cristagalli: (a) inner view of upper valve. (b) inner view of lower valve. Siddiqui & Ahmed : Oysters of Pakistan 115

occasionally be found in semi-protected conditions Iredale43; Crassostrea echinata Quoy & Gaimard46, also but was never observed in exposed environment Thomson31. with heavy wave action. It occurs near high water In this study 2 specimens were examined from mark, in the backwaters and creeks forming only a Gwadar West Bay (size 83-85 mm). Shell inequivalve, single layer of oysters, not allowing a second layer to somewhat oval in outline, lower valve moderately develop on top. This species forms a narrow 0.6-0.9 m deep, rim of the lower valve with 12 to 14 regular wide band if growing on a vertical substratum. It is crenulations, mostly on the posteroventral side. The widely distributed on the coast of Sindh and Balochis- upper valve is rather flat, but with a central hump be- tan11,25,26,27,28. tween hinge and outer lip; rim upraised; lamellar Remarks: This species closely resembles its de- growth lacking. Externally valve is whitish gray with scription given by Awati & Rai11. Asif 28 assigned this a black band inside the upraised rim; internally the species to the genus Saccostrea. He also reported in it colour is white with smudges of dirty yellow and a the presence of highly developed chomata. Awati & purplish black band present on the margin, particu- Rai11, Ahmed27, Ansari & Ahmed44 and this study did larly well marked in the upper valve. In the two not find any denticulation along the margins of the specimens examined the shell near the margins is shell in any specimens. Therefore, following the char- pearly white. The adductor muscle scar is dorsally acteristics of the genus Saccostrea given by Ahmed27 placed, white, and excavated on the hinge side. Inter- and Stenzel32 this species cannot be assigned to the nally the upper valve bears a marginal row of denti- genus Saccostrea. cles which are variably developed, being 38-61 on the ventral side and 31-42 on the dorsal side. Hinge plate Phylum: Mollusca broad tapering towards the anterior side. Class: Bivalvia Habitat: This species was found attached to rocks Subclass: Lamellibranchia at a tidal height of about 1.5 m. This species was Order: Anisomyaria found on the West Bay of Gwadar, Balochistan29. Family: Ostreidae Remarks: Saccostrea echinata is the second most Genus: Saccostrea abundant species on the West bay of Gwadar29. and S. echinata examined during the present study Genus Saccostrea (Dolfuss & Dautzenberg) showed close resemblance to C. echinata from Aus- 31 31 Description of the genus as reported by tralia . Thomson however, reported morphological Stenzel32.“Small to medium-sized, outline variable, variation in this species from the sheltered conditions but divisible into normal ostreiform ecomorphs with such as a slightly folded lip with vague sinulations spatulate or vertical-oval to irregular form and ab- and lamellar growth fringes on the lower valve. normal rudistiform ecomorphs with small, operculi- Species: Saccostrea cucullata (Born) (Fig. 5 B,C) form right valve (upper valve) and large, slender Synonyms: Ostrea tuberculata Lamarck51; Lamy52; conical to cornocupia-like. Left valve (lower valve) 6 11 24 carrying very high, slender ligamental area, and has Ostrea cucullata Hornell , Awati & Rai , Kazmi , Ranjah25 Lamarck51; Crassostrea tuberculata Ah- large attachment area, and grows preferentially on 27 31 33 51 bare rock surfaces or mangroves, carries many rough med , Thomson , Carreon Lamarck ; Crassostrea cucullata Patel & Jetani20,21; Saccostrea cucullata growth squamae and 10 to 30 rough irregular rounded 19 53 dichotomous radial folds extending into small pro- Rao , Ahmed . truding lobes. Umbonal cavity in average very deep. In this study 302 specimens were examined from Both valves with strong chomata, which commonly Paradise Point (size 32-82 mm). The shell is small encircle entire valve. Promyal chamber is present. and subtrigonal, brownish, tainted with purple colour Non-incubatory”. towards the margins and internally the colour of the shell is yellowish with a purple band near the mar- Species: Saccostrea echinata (Quoy & Gaimard) gins. It is distinctly fluted and terminates into sharp (Fig 5 A) crenulations. The upper valve is flat forming a lid, Synonyms: Ostrea mytiloides Cox45; Ostrea echi- crenulated at the margin with distinct anachomata nata Quoy & Gaimard46; Ostrea spinosa Deshayes47; (tubercles or teeth) extending posteriorly almost two Ostrea mordox Cox45; Ostrea nigromarginata Saville- thirds from the hinge. The lower valve is moderately Kent48, Hedley49 (non Sowerby50); Saxostrea gradiva deep with a large recess under the hinge; the margin is

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upraised, fluted, the folds giving rise to sharp crenula- less rounded, situated more towards dorsal side, and is tions. The adductor muscle scar is distinct on the up- green in colour. Anachomata are of small size present per valve, and is variably coloured from white to dark almost throughout the edge of the shell, with corre- purple; on the lower valve the scar is less distinct, a sponding catachomata (pits) in the lower valve. bit excavated on the hinge side and greatly displaced A promyal chamber is absent. The colour of the towards the postero-dorsal side. The colour of the ad- mantle and gills is greenish white. The mantle folds ductor muscle scar sometimes varies on both the are generally without any pigmentation but pale yel- valves. low pigment may be present in many specimens. Habitat: This species was found firmly attached to the rocks, almost blending with them completely and Habitat: This oyster was found on rocky platforms is extremely hard to detach from the substrate. It was at a tidal range of about 0-0.5 m. At Buleji it occurs in found at a tidal height of 2-3 m on the wave-lashed and outside of tide pools on the rocky ledge in the low rocky shores54 and is never seen in protected envi- tide zone and on Keamari seawall. ronment. It remains out of water for very long periods Remarks: This species is being reported for the first of time getting submerged for 4-5 hours per day or time from the coast of Pakistan. The characteristic less. This species has been reported from the wave- features of the shell of this species collected from Ka- lashed rocky shores of Sindh and from the semi- rachi in the present study closely resemble those protected/exposed rocky beach at Jiwani and Gwadar, given for Ostrea nomades from Australia31. Balochistan29. Species: Ostrea folium (Linnaeus) (Fig. 5E): Phylum : Mollusca Synonyms: Ostrea folium Thomson31; Dendostrea Class : Bivalvia folium Sowerby50, Harry34; Crassostrea folium, Rao19, Subclass : Lamellibranchia Rao36. Order : Anisomyaria In this study the preserved samples (10 specimens Family : Ostreidae of size 45-77 mm) collected by Ahmed et al.29 were Genus : Ostrea examined from West Bay of Gwadar, Balochistan. Genus Ostrea (Linnaeus) The external coloration of the shell is bronze pink/red. Internally the shell is opalescent pinkish to copper- 31 Description of the genus as listed by Thomson . coloured, the left valve being slightly concave with a “Shell subcircular, relatively constant in shape within variably folded rim showing weak crenulations. The the species; lower valve shallow, not recessed under upper valve is slightly concave with weakly upraised the hinge, muscle scar subcentral, upper valve may be rim. The hinge plate is narrow. The ligament is brown flat and opercular, or domed and subequal to the with a dark purple patch on either side. The adductor lower, and it may or may not have denticles along the muscle scar is dorsally displaced, and is white to margin laterally to the hinge which fit into pits in the bronze coloured, reniform, and slightly excavated on lower valve. Incubatory forms”. the hinge side. Promyal chamber is absent. Species: Ostrea nomades (Iredale) (Fig. 5 D) Habitat: This species was found on small boulders Synonyms: Ostrea crenulifera (Saville-Kent as in on the West Bay of Gwadar, on the coast of Balochistan. Thomson31); Ostrea nomades Iredale43, Thomson31. Remarks: Thomson31 described various morpho- In this study 673 specimens were examined from logical variations of its shell related to habitat: strong Buleji, and 402 specimens (size 26-97 mm) from flutations in valves, which were loosely attached to Keamari seawall were studied. The lower valve is flat, the substratum in the intertidal zone, but found low it appears slightly concave internally and its rim being degree of flutations which occurred in submerged upraised and folded. The upper valve is convex, conditions or in the oysters, which were firmly at- folded near the rim where it fits into the broad undula- tached to the substratum. The shells of this oyster tions of the lower valve. Externally the shell is green- available to us for study show fewer crenulations and ish white, with a few individuals showing dull yellow foldings compared to those described by Thomson31 . coloration. The internal coloration of the shell is opal- The absence of a promyal chamber and the presence escent greenish white without any well-marked rim. of sperm balls in the histological sections55 qualify The adductor muscle scar is large and green, more or this species for inclusion in the genus Ostrea. Siddiqui & Ahmed : Oysters of Pakistan 117

Species: Ostrea cristagalli (Linnaeus ) (Fig. 5F) The present study shows the occurrence of nine species of oysters namely, Crassostrea gryphoides, Synonyms: Mytilus cristagalli Linnaeus56; Ostrea C. madrasensis, C. belcheri, C. glomerata, Saccostrea cristagalli Lamarck51; Lopha crystagalli Linnaeus56; cucullata, S. echinata, Ostrea nomades, O. folium and Wada57; Torigoe58; Harry34; Crassostrea crystagalli, O. cristagalli on the coast of Pakistan. These belong Rao19, Patel & Jetani20,21, Rao36. to the genera Ostrea, Crassostrea and Saccostrea. In this study preserved samples (2 specimens of Crassostrea gryphoides has been found to be the prin- size 49 and 67 mm) of Ostrea cristagalli collected by cipal edible oyster species of Pakistan. Crassostrea Ahmed et al.29 were examined from West Bay of gryphoides and C. madrasensis are considered here as Gwadar, Balochistan. The external colouration of the separate species. Ostrea nomades and O. cristagalli two shells is light brown. Inner surface of the shell is have been reported for the first time from Pakistan. nacreous and exhibits various shades of brown. The shells are rounded, lanceolate, and have radial folds References feebly developed. Shell margin is deeply crenulated, 1 Abercrombie A, The common marine shells of Bombay, and shows a band of large number of minute tubercles Part I., J. Bomb. Nat. Hist. Soc., 8 (1894) 21-222. (or pustules) all along the entire margin of the upper 2 Comber E, A list of the marine mollusca in the Bombay valve. Hinge is broad, with its ligament being of dark Natural History Society Collection, J. Bomb. Nat. Hist. Soc., brown colour. Adductor muscle scar is oblong and 17 (1906) 207-218. 3 Preston H B, Report on a collection of mollusca from the Cochin brown in colour, excavated on the hinge side and is and Ennur backwaters. Rec. Indian Mus., 12 (1916) 26-39. located more towards the dorsal side. Promyal cham- 4 Hornell J, Note on an attempt to ascertain the principal de- ber is absent. termining factor in oyster spawning in Madras backwaters, Habitat: The specimens were found attached to Madras Fish. Bull., 4 (1910) 1-14. rocks in West Bay of Gwadar, Balochistan. Rao19 5 Hornell J, Note on the identification of the edible oysters of Okhamandal. Report on the marine zoology of Okhamandal found it attached to stones or dead corals. in Kattiawar, 1916, pp. 32. Remarks: There is the presence of a series of small 6 Hornell J, The edible molluscs of the Madras Presidency, round granules (minute tubercles or pustules) all Madras Fish. Bull,. 11 (1918) 10-16. along the inner margin of the upper shell valve. Patel 7 Hornell J, The common molluscs of South India, Madras 20,21 Fish. Bull., 14 (1921) 97-215. & Jetani also reported the occurrence of such 8 Hornell J, The study of Indian molluscs, J. Bomb. Nat. Hist. granules in their specimens collected from the Gulf of Soc. 48 (1949) 750-774. 32 Kutch. Stenzel identified this species as Lopha 9 Hornell J, Indian molluscs, (Bomb. Nat. Hist. Soc., Bombay) cristagalli, having sharp radial plicae, which are not 1951, pp. 58-60. present in the specimens at hand. Stenzel32, however, 10 Prashad B, Report on a collection of Sumatran Molluscs from fresh and brackish water. Rec. Indian Mus., 12 (1921): reported the presence of a band of tubercle-shaped 501-502. anachomata all along the margin of the right valve in 11 Awati P R & Rai H S, Ostrea cucullata (The Bombay Oys- the Indo-Pacific specimen obtained from Guam. ter), Indian Zool. Mem., 3 (1931) 1-107. Hornell5,7 identified this species as Ostrea cucul- 12 Rai H S, The shell fisheries of the Bombay Presidency. Re- lata. Satyamurti15 described C. cucullata as port on the Bombay Natural History Society’s Survey, J. Bombay Nat. Hist. Soc., 35 (1933) 826-847. C. cristagalli. This species, however, stands out from 13 Subrahmanyam T V, Karandikar K R & Murti N N, The ma- all other Ostrea as Lopha cristagalli, owing to the rine Pelecypoda of Bombay, J. Univ. Bombay, New Ser., 17 presence of the 2-3 rows of a large number of minute (1949) 50-81. 14 Patil A M, Study of the marine fauna of the Karwar Coast granule-like tubercles (pustules ⎯ a term used by 32 and neighbouring islands. Bomb. Nat. Hist. Soc., 51 (1954) Stenzel ) all along the margin of the right valve of 29-41. the shell. These tubercles, which are barely visible to 15 Satyamurti S T, Mollusca of Krusadai Island, in: The Gulf of the naked eye, do not have corresponding pits in the Mannar-II-Scaphopoda, Pelecypoda and Cephalopoda,. lower valve, and differ considerably in form and (Madras Govt. Mus., N. S.- Nat. Hist. Sect.) 1956, pp. 1-202. number from the anachomata present in oysters of the 16 Kundu H L, On the marine fauna of the Gulf of Kutch, Part III-Pelecypoda, J. Bomb. Nat. Hist. Soc.., 62 (1965) 84-103. genus Saccostrea (such as S. cucullata ) and Ostrea. 17 Durve V S, On the nomenclature of two Indian backwater We have found Ostrea-like eggs and sperm balls in oysters,. J. Mar. Biol. Ass. India, 9 (1967) 173-178. the histological sections of the gonads of this species, 18 Durve V S, Malacological differences between the oysters, which is indicative of the fact that it is a species of the Crassostrea gryphoides (Schlotheim) and Crassostrea ma- larviparous oyster genus Ostrea55. drasensis (Preston), Indian J. Fish., 20 (1974) 624-625. 19 Rao K S, Taxonomy of Indian oysters, Bull. Cent. Mar. Fish.

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